No problem, take the time you´ll need. To my knowledge, the Miata gearboxes are all the same of NA/NB/NC, even the 6 speed seemes to be just upgraded from the 5 speed. If the parts might be suitable, I rather would have to source the correct box in Germany for investigation. Several different specs might be confusing to find the right one. But I have to be patient, by no means it´s certain to be accepted by the dimensions.

OK, got one of the Hewland dog rings from the shop and did a 5 minute solidworks model and drawing, leaving out most chamfers and the like. As I noted on it, I'm measuring from a used part and if you're doing serious design work, I seriously urge you to get new parts to work from.

Thanks a lot for the sketch, looks well-known. Unfortunately,I have currently nothing more to do with dog boxes on the job. Probably you can find out some more info about the Miata box, mainly at first the centerline distance of the shafts and maybe the design of the synchro hub and sliders of 1/2 gear. If I get it well done anywhere, I would prefer a synchro box due to the traffic use without issues and if would ever sell the whole stuff it´s simpler to offer a synchro box with what everyone is familiar in using. The gearchange of close ratio synchro boxes works as well very quick due to the small difference in gearwheel speeds. No more a big difference to the dog engagement, without the wear issues.

All right, I went and sorted through my pile of parts. First off, a brief history lesson... there's 4 performance Mazda transmissions (well, maybe 5 now, but let's ignore the really new one) The "rib case" is a bottom loading older 4-speed with a detachable bellhousing that they later used for the turbo RX-7 transmissions. The "smoothcase" is a 5-speed with the bellhousing integrated into the case that loads the gears from the rear - it's used in all sorts of things, but most crucially the non-turbo RX-7s later than 1980 or so, and all the 5-speed MX-5s up to at least 2015. There's the Aisin 6-speed available as an option in the 1999-2005 MX-5s and also used in the 2004-2009 RX-8s, plus versions of it are used in the Toyota GT86 and some other cars, which has an integral bellhousing and loads the gears from the rear also. And there's an in-house 6-speed used in the 2006-2015 MX-5 and RX-8s later than 2009... and also the new Fiat 124 I think, which has an integral bellhousing and loads the gears from the top.

As far as I know, few if any parts interchange between the gearboxes. I bring this up because I want to be clear what I'm talking about.

The gearbox I have the most experience with is the "smoothcase" 5 speed from the 1980s RX-7s. I sorted through my parts, found a bearing plate and measured the center to center between the shafts to be 68.5mm. From the designs of the various transmissions I am sure that this is the smoothcase dimension. The problem is that some 6-speed parts got mixed into the box - I want to confirm for sure which parts are which before I make drawings of them. The later MX-5s also might have made some changes to the synchronizers. I'll dig through my parts catalogs to make sure of that too.

Seems to be a bit confusing for an outsider.....looks like there are no correlations between the boxes to mine. I found now a guy from Romania who is very familiar with Renault boxes and it seems that we can find a solution.
But anyway, lots of thanks to you for your effort ! Hopefully I can help as well if required.....